From DE 1 107 471 A there is known a multistage controllable pressure reducing valve for reducing fluid pressure, which is intended primarily for feed water or the injection of water vapor. The pressure reducing valve has a housing with a longitudinally extending inner chamber, an inlet opening and an outlet opening and with a plurality of annular walls which are arranged between the inlet opening and the outlet opening, are separated from one another by housing recesses and contain coaxial valve openings. Extending through the valve openings is a spindle guided in the valve openings in the manner of a piston and including milled recesses extending in the longitudinal direction of the spindle and determining the control characteristic. The known configuration of the pressure reducing valve is only suitable for use with pure water since particles of dirt may accumulate in the milled recesses and impair the movability of the spindle. Furthermore, the longitudinally extending milled recesses limit the control range appreciably.
Other pressure reducing valves as known, for example, from DE 32 15 224 C2 and DE 37 44 730 C1, have a valve piston arranged in a housing bore for axial movement and including annular recesses capable of exposing valve openings opening radially into the valve bore. To reduce energy by vortical circulation, the valve openings in the housing are additionally surrounded by annularly arranged throttling devices. These valve configurations are likewise only suitable for use with pure liquids which cannot clog the narrow throttle openings.
Furthermore, there is known from EP 0 744 527 B1 an apparatus for the transmission of data present above ground to a data receiving device below ground in a borehole, wherein a bypass conduit discharging into the mud pit and including a shut-off device actuatable by a fluid pressure drive is connected to the supply conduit of a mud pump driven at a constant output. Connected downstream of the shut-off device is a throttle which reduces the pressure energy of the drilling fluid escaping from the supply conduit with the shut-off device open. To transmit data to a downhole data receiving device, the shut-off device is actuated in the opening direction during uninterrupted drilling operations, whereby the flow rate of the drilling fluid in the drill string is reduced. The variation of the flow rate over time produces a signal capable of being detected by the downhole data receiving device. Depending on the manner of actuation and design of the shut-off device, a pulse-like pattern can be imposed on the volumetric flow variations, and by variation of the pulse pattern it is possible to produce as a binary sequence coded signals which serve to control components of the downhole system.
However, the reduction to practice of the above-described method of signal generation is fraught with major difficulties because, depending on the operation of a drilling rig, the conditions in the supply conduit of the mud pump may vary widely. At shallow drilling depths the drilling fluid pressure on the drill head is comparatively low as a rule, while the flow rate is quite high. In order to achieve flow rate variations suitable for signal transmission, it is therefore necessary to direct a comparatively large bypass flow into the pit at a low pressure. At large drilling depths the drilling fluid pressure on the drill head is necessarily very high whilst the flow rate is relatively small as a rule. An attendant problem is to let only a limited amount of volumetric flow escape through the bypass conduit and into the mud pit, in spite of the high pressure, which makes it necessary for a corresponding pressure energy of the branched off bypass flow to be reduced. Intermediate states between said extremes must also be negotiated by the shut-off device provided for signal generation.
Another problem presenting itself is the unavoidable loading of the drilling fluid with abrasive solid particles which, on the one hand, may result in severe wear of the shut-off and throttling device and, on the other hand, may also cause clogging and eventual failure of the shut-off and throttling devices.